Solid state radiation detectors are known in the art, and provide a useful mechanism for detecting radiation. Such detectors have evolved from detectors using surface barrier electrodes, to ion-implanted electrodes.
Descriptions of such detectors and their uses may be found in applicant's article "A Proposed VLSI Pixel Device for Particle Detection", Nucl. Instr. and Meth. A275, 494 (1989), A342 59-77 (1994), and in U.S. Pat. Nos. 4,593,381, 5,237,197, 5,355,013, 5,461,653 and 5,465,002, among other references. FIG. 1 depicts such a prior art radiation detector 10, which is more fully disclosed in U.S. Pat. No. 5,237,197 (in which applicant herein is co-inventor). In FIG. 1, a detector array 10 includes a preferably lightly doped P-type charge depletable substrate 20, having first and second surfaces 30 and 40 spaced-apart by a substrate thickness L of perhaps a few hundred microns. Substrate thicknesses in this range provide good sensitivity for collecting radiation-generated charge from within the substrate, as well as providing acceptable voltage break-down levels, and protection from radiation damage.
Adjacent the first substrate surface 30 voltage-biasable doped well regions 50 of preferably N-type material are formed. Buffer well region 55 is formed of N-type or P-type material, depending upon the nature of the circuitry 60 in this well region. Well regions 50, 55 preferably are sufficiently highly doped to act as an electrostatic shield for underlying regions of the detection device. Electronics 60 may be fabricated within buffer well region 55.
Also adjacent first substrate surface 30 and separated from each other by the N-type well regions 50 are formed spaced-apart collection electrodes 70, preferably made from highly doped P-type material. Preferably the gate lead of one (or more than one) metal-oxide-semiconductor ("MOS") transistor 80 is coupled to each collection electrode 70. The lower surface of the substrate includes a preferably heavily doped N-diffusion region 90, beneath which is an electrode (not shown), and isolation regions 100. Of course, the conductivity types of the materials used to form detector 10 could be reversed, e.g., substituting P-type for N-type and vice versa.
One collection electrode 70, its associated MOS device 80, and indeed the associated underlying semiconductor structure may collectively be termed a "pixel", and the terms pixel and detector may be used interchangeably. It is seen from FIG. 1 that P-type collection electrodes 70 and P-type substrate 20 form a plurality of PN diode junctions with the N-type well regions 50 adjacent the first surface.
In practice, a well bias voltage of many volts is coupled between the collection electrode regions and bottom regions and N-doped well regions. The resultant electric fields extend from the second surface 40 toward and to the first surface 30. The resultant depletion region extends through the perhaps 300 .mu.m thickness of the substrate, whereupon a plurality of P-I-N diodes are formed by P-type collection regions 60, intrinsic substrate region 20, and N-type region 90.
The biasing causes force lines to emanate from the N-diffusion region 90 through the substrate thickness and focus upon the P-type collection electrodes 70. Incoming radiation (not shown) releases charge within the substrate, which charge is focused by the force lines and caused to be collected by the electrodes 70. As noted, N-wells 50 further serve as a Faraday shield for the array of pixels in structure 10. As noted, well regions 50, 55 can also serve as areas in which electronics are fabricated. Unfortunately, CMOS electronics that require wells of both dopant conductivity types can present a problem. Such CMOS electronics can be accommodated in the area over the active detection region, providing wells of like-conductivity type as the collection electrodes are implanted completely within wells of the opposite conductivity type. Understandably, if same-type wells were to be formed directly on the depleted silicon substrate, the wells would collect ionization charge on the substrate, which charges would not be collected and detected by the collection electrodes and assorted circuitry.
As described in the U.S. Pat. No. 5,237,197, detector 10 can nonetheless function reasonably well because the wells surrounding the collection electrodes were doped with opposite type dopant and were back-biased relative to the collection electrodes. This configuration caused electric field lines to be directed to carry one sign of ionization charge from the substrate and the well to the collection electrodes. Like-signal wells, needed for CMOS electronics, were placed along the structure edges, beyond the sensitive detection area. Even though only perhaps 10% of upper surface 30 may be covered by collection electrodes, efficiency in the sensitive region is extremely high with more than 99.99% of the radiation-induced charges being collected by electrodes 70. The collection electrodes preferably are uniformly distributed in a two-dimensional array on the surface, to provide resultant uniform array sensitivity and spatial resolution.
Once the radiation-induced charge has; been collected by the collection electrodes 70, the transfer to an associated MOS device(s) can be rapid as the distance is now but a few microns. Further, because there is small capacitance (C) at the MOS gate, the charge (q) developed by the incoming X-ray radiation can produce a substantial voltage signal (v), since v.apprxeq.q/C. Electronics 60 may be used to signal process the charge associated with the MOS devices. For example the collected charge at a MOS gate may be used to modulate readout current caused to flow through the MOS device. Such readout may be made on an addressable row-column basis.
As noted, radiation detection sensitivity for prior art detector 10 can be very high. But radiation-induced charge cannot be detected until it has been collected by the surface-located collection electrodes 70. Unfortunately the collection or drift path that released charge must traverse before being collected can be very long, e.g., comparable to the few hundred micron full substrate thickness. Of course should some radiation-generated charge happen to be released closer to the collection electrode surface, collection of the charge can occur in a shorter time. In practice, prior art: detectors using two-dimensional electrodes such as shown in FIG. 1, or the common silicon strip technology that preceded what is shown in FIG. 1, may take upwards of 25 ns for charge-produced signals to return to a baseline level from a peak. Of course, amplifier delays may extend this time even further.
Attempting to reduce radiation detection time by using a thinner substrate is counter productive because thinner substrates have shorter tracks, and therefor less signal charge. Also, thinner materials can break more readily during fabrication. It would also be desirable to provide a detector structure, that is kept small in size in the presence of radiation damage, requiring a smaller voltage magnitude to achieve depletion, while still preventing so-called bulk type-reversals. Finally, in many detection environments it is necessary to continuously refrigerate the detector, even for maintenance.
U.S. Pat. No. 5,889,313 provided a sensitive solid state radiation detector having such improved detection response times, with good voltage breakdown, without requiring excessively high depletion voltages, while still exhibit good radiation damage resistance characteristics. The resultant detector permitted implementation as a monolithic combination of collection electrodes and CMOS electronics, without thereby hindering collection of released charge and functioned without requiring operation at low temperature to prevent radiation damage effects.
Notwithstanding their improved performance, such detectors (not unlike other prior art detectors) would not typically be fabricated such that substantially the entire surface of an IC chip could be detection-active, literally from chip edge-to-edge. It is normal to leave a wafer-area safety margin surrounding detectors, especially planar detectors, for several reasons. The depletion region and associated electric field tend to bulge out from the last electrodes (e.g., those nearest a detector edge). When the detector is sawed from the IC wafer, the passivated saw cuts (and any resulting chips or cracks in the wafer edge) must not reach or interfere with such bulges and thus a margin of detection-inactive area is left between the end detectors and the raw edge of the IC. It is also common IC fabrication practice to provide guard and/or voltage dropping rings around active regions including detectors. Fabrication such rings requires IC area that could otherwise perhaps be made available for active detection. Presenting a larger overall detection area by combining multiple detector arrays typically requires multiple overlapping detectors. Accommodating the overlap area also requires inactive area on the detector IC. Thus, due to a variety of reasons, the detection-inactive (e.g., dead) area on a detector IC may represent 10% to 20% or more of the IC area that might otherwise be used for active detection.
Understandably, it would be desirable to fabricate a detector IC that was detection-active from edge-to-edge. Not only would such an IC provide a greater detection area for a given IC size, but combining many such ICs in a preferably tiled planar array (although non-planar could be used) could be carried out without dead space surrounding each individual detector IC. Indeed, it would also be highly useful if the vertical chip edge surfaces could also be rendered detection sensitive.
Regardless of whether edge-active detection is available, it would be useful for a radiation detector to output a larger signal. Such an enhanced output signal detector would find use in applications where existing signal strength is too marginal, or requires overly sophisticated amplification and signal processing. Preferably such increased signal output: mode should be accomplished without substantial modification to the detector, preferably by operating the detector in a different bias regime.
Although conventionally radiation detectors, including those described in U.S. Pat. No. 5,889,313 have electrodes formed in a semiconductor substrate, it would be useful to provide detectors formed in a substrate that was not necessarily even semiconducting, including for example, insulating or near-insulating semiconductors. Rather than being formed with P-I-N structures, such devices would be characterized as having conductor-insulator-conductor structures and would have essentially no leakage currents.
As noted in U.S. Pat. No. 5,889,313, it is often necessary to interconnect different ICs, perhaps a detector IC and a second IC containing electronics, perhaps readout circuitry. In practice it can be difficult to mechanically and electrically make such interconnections. Bump bond interconnections often involve the presence of indium or gold, whose mobile atoms can detrimentally affect MOS device performance, and bond leads. Bump bonding chips cannot be placed in intimate contact with each other, which gives rise to higher interconnect capacitances. Further bump bonding yields are typically in the 90% range. What is needed is a method whereby such mechanical and electrical interconnections can be made with very high reliability, preferably using IC fabrication equipment normally found in a fabrication site.
Finally, it can be useful to provide ICs including electronics for three-dimensional detectors with an oversurface layer or film that can protect the underlying structure, especially for use in a hostile environment. Preferably such layer or film should be formed at room temperature using fabrication techniques and equipment that are presently available. Preferably such film would also lend itself to micromechanical devices, as well as sensors and other ICs. Anisotropic silicon wet etching is widely used in the industry, especially for bulk micro-machining and micro-electromechanical systems (or "MEMS"). While such etching an inexpensively selectively remove large volumes of silicon, a mask layer of silicon dioxide or silicon nitride is required. Such masking requires several steps extending hours while these materials are grown or deposited, often at high temperature. Further, different anisotropic etchants are selective to different masking layers, e.g., KOH deep etching requires a silicon nitride mask). Thus, specific etchants must be paired with specific masking layers, which contributes to manufacturing complexity and/or to the manner in which the component parts can be made. Further, aluminum and polysilicon are attacked by most common anisotropic etchants, and must be protected during a silicon etch, after which windows must be formed in the masking layer. Finally, it is often desired to etch only one side of a wafer, during which process the other wafer side must be protected, either mechanically or by deposition and masking an additional layer on the side to be protected, and then removing the protective layer after bulk etching on the other side.
The present invention provides such a detector and mode of detector operation.